Field of the Invention
The present invention relates to an energy cable. In particular, the present invention relates to a cable for transporting or distributing electric energy, especially medium or high voltage electric energy, said cable having at least one thermoplastic electrically insulating layer.
Description of the Related Art
Cables for transporting electric energy generally include at least one cable core. The cable core is usually formed by at least one conductor sequentially covered by an inner polymeric layer having semiconductive properties, an intermediate polymeric layer having electrically insulating properties, an outer polymeric layer having semiconductive properties. Cables for transporting medium or high voltage electric energy generally include at least one cable core surrounded by at least one screen layer, typically made of metal or of metal and polymeric material. The screen layer can be made in form of wires (braids), of a tape helically wound around the cable core or a sheet longitudinally surrounding the cable core. The polymeric layers surrounding the at least one conductor are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197.
To address requirements for materials which should not be harmful to the environment both during production and during use, and which should be recyclable at the end of the cable life, energy cables have been recently developed having a cable core made from thermoplastic materials, i.e. polymeric materials which are not crosslinked and thus can be recycled at the end of the cable life.
In this respect, electrical cables comprising at least one coating layer, for example the insulation layer, based on a polypropylene matrix intimately admixed with a dielectric fluid are known and disclosed in WO 02/03398, WO 02/27731, WO 04/066317, WO 04/066318, WO 07/048,422, and WO 08/058,572. The polypropylene matrix useful for this kind of cables comprises polypropylene homopolymer or copolymer or both, characterized by a relatively low cristallinity such to provide the cable with suitable flexibility, but not to impair mechanical properties and thermopressure resistance at the cable operative and overload temperatures. Performance of the cable coating, especially of the cable insulating layer, is also assisted by the presence of the dielectric fluid intimately admixed with said polypropylene matrix. The dielectric fluid should not affect the mentioned mechanical properties and thermopressure resistance and should be intimately and homogeneously admixed with the polymeric matrix.
More and more stringent long term cable performances, especially for medium and high voltage application, are increasingly requested by international standards, both for safety and economical reasons.
One of the major phenomenon possibly accelerating the cable ageing and shortening the lifetime thereof is the so called “water treeing” caused by moisture penetration into cable layers, especially into semiconductive and insulation layers. The moisture penetration and the consequent water trees are often eased by voids, defects (even micro-defects) and contaminants present in the layers.
In the field of energy cables having as insulating layer a crosslinked polyolefin composition, such as crosslinked polyethylene (XLPE) or crosslinked elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, it is known, in order to prevent water-treeing and the harmful consequences thereof, to add to the material forming the insulating layer small quantities of additives commonly known as “water tree retardants”, which should be able to prevent the water tree growth and to promote the dielectric strength retention and, as a consequence, the longevity of the cable.
Various kinds of water tree retardants are known. For example, U.S. Pat. No. 4,370,517 aims to suppress the generation of water trees in the insulator of electrical machinery such as cables, etc. by incorporating into the insulator surrounding an electrical conductor a specified ester group content. Various sources of ester groups can be used. Namely, salts or esters of fatty acids can be incorporated into the polyolefin. As the fatty acid, stearic acid, sebacate acid or adipate acid can be used, but those of ordinary skill in this art will find that any fatty acid and preferably those having from 6 to 31 carbon atoms are suitable, the essential point being the presence of the ester group itself in the polyolefin composition. Examples of suitable fatty acid salts are cadmium stearate, lead stearate, zinc stearate, and lithium stearate. Suitable esters are esters of the above fatty acids and a polyol or glycol. Preferred polyols contain 5 to 6 carbon atoms. The glycol, e.g., polyethylene glycol may contain 2 to 46 carbon atoms. Specific examples of the esters include polyethylene glycol distearate, polypropylene adipate, polypropylene sebacate, stearic acid monoglyceride, sorbitan trioleate, sorbitan tristearate, polyoxyethylene sorbitan tristearate, etc.
Alternatively, according to the same U.S. Pat. No. 4,370,517, the ester group can be introduced directly into the polyolefin polymer chain through the use of a predominantly polyolefin based copolymer containing as a comonomer a monomer containing an ester group, for example, ethylene-vinyl acetate copolymer (hereafter EVA), ethylene-ethyl acrylate copolymer (hereafter EEA), ethylene-methyl acrylate copolymer (hereafter EMA), ethylene-methyl methacrylate copolymer and mixtures of each of them.
U.S. Pat. No. 4,305,849 relates to a polyolefin composition far less vulnerable to the deterioration that occurs as dendroid voids when employed as an electrical insulation of high-tension power cables which are operated under water. In particular this document teaches a polyolefin composition for electrical insulation which comprises polyolefin or crosslinked polyolefin and a small quantity of high-molecular weight polyethylene glycol. High-molecular weight polyethylene glycol which characterizes this invention was selected from a series ranging from 1,000 to 20,000 in the molecular weight.
EP 0 814 485 relates to an improved electrical insulation composition that maintains high breakdown strength over time, by minimizing the formation of water trees. Improved water treeing resistance can be obtained by using additives wherein the hydrophilic portion is made up of polar units, preferably linear alkene oxide units and, more preferably, ethylene oxide. In a preferred embodiment the compatible portion is made of aliphatic, aromatic or low polarity units; preferably a hindered alkene oxide, and, more preferably the compatible portion is propylene oxide. Most preferably, the additive is an ethylene-oxide/propylene-oxide block copolymer.
WO 2010/072396 relates to a process for preparing polymer composition by blending a masterbatch which comprises at least one water tree retardant additive and a polyolefin and the use of said polymer composition for preparing an article, preferably a cable. Said masterbatch comprises preferably one or more water tree retardant additive(s) selected from the group comprising, among the other, a polyethylene glycol(s), an amido group containing fatty acid ester(s), an ethoxylated and/or propoxylated fatty acid(s).
Though mentioning a variety of polymeric materials suitable for insulation, or semiconductive layers, none of the prior art documents deals with the problem of inhibiting the growth of water trees and with the improvement of longevity of an electric cable having a layer, especially the insulation layer, based on a polypropylene compound including a dielectric fluid.
The presence of a dielectric fluid, even if intimately admixed within the polypropylene matrix, could constitute a weakening point for the insulating layer from the water-treeing point of view, especially in cables designed for medium or high voltage current transport.